An exhaust system conducts hot exhaust gases generated by an engine through various exhaust components to reduce emissions and control noise. The exhaust system includes an injection system that injects a diesel exhaust fluid (DEF) or a reducing agent, such as a solution of urea and water for example, upstream of a selective catalytic reduction (SCR) catalyst. A mixer is positioned upstream of the SCR catalyst and mixes engine exhaust gases and products of urea transformation. The injection system includes a doser that sprays the urea into the exhaust stream. The urea should be transformed as much as possible into ammonia (NH3) before reaching the SCR catalyst. Thus, the droplet spray size plays an important role in reaching this goal.
In one known configuration, the mixer includes an outer housing with an opening that receives the doser. A cone is aligned with this opening to provide a widening spray area into the internal cavity of the outer housing. The widening spray area facilitates a more thorough distribution of spray droplets over a larger area as the spray enters the exhaust gas stream.
The industry is moving towards providing more compact exhaust systems, which results in reduced volume of the system. Systems that spray larger size droplets may not be able to provide adequate transformation of urea when used in more compact system configurations. As such, smaller droplet size dosers are required for these more compact configurations.
The smaller the droplet size, the more effective the transformation into ammonia is, due to the increased surface contact area. However, the spray generated by small droplet dosers is very sensitive to recirculation flow. Typically, an area located at a tip of the doser has a vortex of recirculating flow. This vortex pushes the spray droplets towards the walls of the mixer and onto the tip of the doser, which creates deposit initiation sites. In configurations that utilize a cone, deposit initiation sites have been found on the wall of the cone. The deposits in these sites build up over time and can adversely affect system operation. For example, there may be a lower ammonia uniformity index, there may be an increased pressure drop across the mixer, or they may be a peak of ammonia emissions during active diesel particulate filter (DPF) regeneration.